Its hard to choose, interlingua, esperanto, lingwa de planeta, ido, uropi, lingua franca nova.
Personally I think its better to develop an international sign language.
It's much easier to learn as you dont need to learn new spellings or alphabets (the sign for "a" can also be greek alpha for instance), or germanic or slavic or latin roots, it is only actions added to your own local word, so literally verbal monoglots can speak to any other monoglot. No matter how thick their accent is.
Plus it would greatly expand the deaf communities international reach, would not exclude the deaf, and I like the idea of children learning an international auxiliary language that can also help the hard of hearing.
And if you've ever been in a loud bar or nightclub or restaurant in a foreign country in europe it would make ordering much easier. Obviously unfortunately it does somewhat exclude disabled people with reduced mobility and wouldn't work on the phone, but we all have videocalls now and we shouldn't be on the phone while driving anyway. Also it's quite fun. I've started in French Sign Language (Elix app on ios app store) and am pushing to have lessons for my colleagues as we work in medical training and I think its nuts its not offered let alone encouraged or compulsory.

So I say we should develop Euro Sign Language.

I mean we all use the middle finger already, lets expand on that.